Add option to customize keycloak themes

[viniciusdc]

Reference Issues or PRs

closes #1980

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Any other comments?

[viniciusdc]

This issue has been on my plate since Qhub and we've wanted to address it for a while, but I only recently migrated it to the current source code.

[viniciusdc]

How to test this PR:

• Create a public github repo, containing a folder with the name to your theme (e.g demo);
• Inside such folder copy over the contents from https://github.com/keycloak/keycloak/tree/main/themes/src/main/resources-community/theme/base or use my-cool-theme-name example from https://github.com/keycloak/keycloak-quickstarts/tree/latest/extension/extend-admin-console.
• In your nebari-config.yaml, make sure to properly pass the URL for you github repo under the repository and branch fields, introduced in this PR
• After deploying, go to the Keycloak UI and select your new theme

[marcelovilla]

@viniciusdc is this ready for review? If yes, let me know and I'll test so we can merge it!

[viniciusdc]

Hi @marcelovilla yes this is ready for review, I still need to open a doc PR for this, but in the meantime if you can test, that would be nice

[marcelovilla]

@viniciusdc sorry that I haven't gotten around to test this PR but I'll do that between today and tomorrow